The use of powdered activated carbon (P.A.C.) as a filter medium in water clarification and coagulation applications is well known. For instance, in Rice et al, U.S. Pat. No. 3,252,899, powdered activated carbon is added to the water to be clarified and is passed through a separation bed. The activated carbon particles are retained in the separation bed and provide an increased surface area upon which organic materials may be adsorbed.
Similarly, "Measurement of Trihalomethane and Precursor Concentration Changes," Stevens et al, Journal AWWA, pages 546-54, October, 1977, reports that trihalomethane removal efficiency may vary widely with time in service of granular activated carbon filters.
In U.S. Pat. No. 3,917,821 (Manes), activated carbons are combined with thickening agents such as sodium carboxymethyl cellulose, dextrans, xanthans, guar gums, etc. to form anecdotal compositions and methods. However, the particular activated carbons disclosed by Manes include Pittsburgh activated carbons which are derived from coal precursors or Nuchar carbons which are derived or based on paper by-products. The disclosure indicates that the Pittsburgh type coal-based activated carbons are preferred (col. 3, lines 13-18).
In contrast to the Manes diclosure, the powdered activated carbons which are herein used in the claimed compositions are derived from wood. That is, these particular wood-based carbons of the present invention are made by the destructive distillation of wood. As is known in the art, these wood-based carbons are prepared by driving off the volatile constituents from the wood precursor and then oxidizing the residue. Comparative stability studies have shown that a wood-based powdered carbon exhibits superior stability in comparison with the tested coal based activated carbons.
Other prior art materials which disclose varying uses of powdered activated carbon include U.S. Pat. Nos. 4,043,904 (Takeda et al); 4,239,865 (Tarao et al); 4,320,011 (Sato et al); Chem Abstract 87:90304e (1977); Chem Abstract 89:117244m (1978); Chem Abstract 94:157877e (1981); and Chem Abstract 89:117260p (1978). Of further possible interest is "Industrial Gums Polysacchar1des and Their Derivatives", Second Edition, Whistler, Academic Press 1973, pp. 658-659.
Although the use of powdered activated carbon is well known in the art, numerous problems have arisen when the product has been shipped in dry, particulate form. Dusting problems can occur at usage locations due to the propensity of the P.A.C. to rapidly form airborne particulates. As such, potential health concerns are raised.
Additionally, in those situations in which the end-user must prepare a P.A.C. containing slurry prior to its use, a large holding vessel is needed in order to prepare the slurry.
Accordingly, there is a need in the art to provide a P.A.C. containing slurry product which can be shipped and even stored for reasonable time periods without substantial gellation or phase separation. Of especial importance is the need for such a slurry product which can easily be handled and directly pumped from its container to the desired end-use location.